Multiple antenna techniques play an important role in modern wireless communication systems to provide improved system performance, including increased capacity and coverage, as well as improved service provisioning. One challenge with the implementation of multi-antenna techniques is the acquisition of channel state information (CSI) at the transmitter or the receiver. In general, the channel can be estimated through a predefined training sequence, which is often referred to as reference signal. Taking the downlink transmission as an example, the base station should transmit reference signals to mobile terminals so that the channel matrix can be estimated at the receiver side. With this estimated channel matrix, coherence demodulation can be carried out. Consequently the potential beamforming gain, spatial diversity gain and spatial multiplexing gain can be obtained. In addition, the reference signals can be used for channel quality measurements to support link adaptation. In the case of orthogonal frequency division multiplexing (OFDM) transmission, a straightforward design of reference signal is to insert known reference signals into the OFDM time-frequency grid.
In the 3GPP specifications 3GPP TS 36.211 V8.3.0 (2008-05), “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation”, 3GPP TS 36.212 V8.3.0 (2008-05) “Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding” and 3GPP TS 36.213 V8.3.0 (2008-05) “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures”, two kinds of downlink reference signals are defined, i.e. the cell-specific reference signal and the UE-specific reference signal. (The above mentioned 3GPP specification refers to LTE release 8.)
Up to four cell-specific reference signals are defined corresponding to four antenna ports 0 to 3, as shown in FIG. 1, targeting to support codebook based multiple streams spatial multiplexing transmission
A codebook is a predefined finite set consisting of a number of precoding matrices with different ranks. In codebook based precoding, the user equipment (UE) will first estimate the channel matrix based on the multiple cell-specific reference signals and then the UE applies an exhaustive search over all precoding matrices and reports the preferred precoding matrix indicator (PMI) to the base station, referred to as eNodeB in LTE, under certain criterions, e.g., maximizing system throughput. Note that the PMI can be overridden by the eNodeB. It should be noted that the channel matrix refers to the precoded channel information by multiplying precoding matrix, i.e. Y=H*W, where Y is the precoded channel matrix, H is channel matrix and W is precoding matrix.
However, only one UE-specific reference signal on antenna port 5 is defined, which is transmitted only on the resource blocks upon which the corresponding physical downlink shared channel (PDSCH) is mapped, as shown in FIG. 2, targeting to support non-codebook based single stream beamforming transmission
In non-codebook based precoding, the precoding weight matrix applied both on UE-specific reference signals and the data signals is not retrieved from the codebook set but is directly calculated by the eNodeB in terms of some criterions, e.g., eigen-decomposition based or direction of arrival based criterion. In TDD (time Division Duplex) system, due to channel reciprocity, non-codebook based beamforming/precoding can reduce further uplink feedbacks and improve beamforming gain.
In the downlink transmission of the LTE system, both codebook based precoding and non-codebook based beamforming/precoding are supported for up to 4 transmit antennas.
The transmission mode switch between codebook based multiple streams spatial transmission is semi-statically configured via higher layer signalling.
However, future communication systems, e.g., LTE-Advance, are likely to employ more transmit antennas in order to reach more aggressive performance targets. Especially, for systems with e.g., 8 transmit antennas, extension of current codebook based precoding is needed from precoder and reference signal perspective.
On the other hand, non-codebook based multiple stream beamforming is considered to be one of the most important candidate technologies for future wireless communication systems. This enhanced multi-antenna technique can simultaneously make use of the beamforming gain to improve receive signal-to-noise-ratio (SNR) as well as multi-layer transmissions to improve the peak data rate. Compared to codebook based spatial multiplexing transmission, non-codebook based multi-stream beamforming has potentially the advantages of signalling overhead reduction and performance improvement.
However, in order to support non-codebook based multi-stream beamforming, it is necessary to define multiple UE-specific reference signals with least effort, especially based on existing antenna port 5 and the UE-specific reference signal as defined in LTE release 8.